The multicast technology effectively solves the problem of “single-point sending & multi-points receiving” and realizes efficient point-to-multipoint data transmission in the network and therefore highly saves network bandwidth and reduces network load. As a communication method that is parallel to unicast and broadcast, the significance of multicast is not limited thereto. What's more, by utilizing multicast characteristics of the network, it can conveniently offer some new value-added services (VAS) , including Internet information services such as live online, digital TV, network TV, distance learning, telemedicine, network radio station and real-time video conference, and the like.
As regards to routing equipments, the network multicast protocols mainly include intra-domain multicast protocol such as PIMSM, PIMDM, DVMRP and the like, and inter-domain multicast protocol such as MSDP and the like. Characteristics of the multicast protocols that they are generated as required or maintain the multicast routing according to requirements can theoretically save network bandwidth and also keep the multicast protocol operating normally under complicated dynamic network environment. However, in practical applications there exists problem of relatively low application timeliness.
Taking the current IPTV as an example, the data bearer routing protocol of IPTV is a multicast protocol. However, the subscribers of IPTV require that the immediacy of watching programs in IPTV should not have an evident difference compared to traditional TV, which means that the terminal user can receive the requested program soon after sending out an application request. Moreover, when the subscriber is switching between different IPTV programs, the switching delay should be also within the range that human beings can bear. The above demands, when reflected onto the multicast protocol, require the multicast protocol to ensure that the delay from initiating a specific multicast application demand to receiving multicast service data by the starter can be as short as possible. The present multicast protocols can not meet these requirements in the applications.
In a routing environment with more than five hops, we tested IPTV forwarding based on PIMSM multicast protocol and its delay time is 1-7 seconds, and when the network environment gets more complicated, the delay time will be longer. The reason that leads to the delay can not be totally avoided by the multicast protocol itself. Since no matter either the multicast protocols DVMRP and PIMDM with “Broadcast and Prune” multicast routing formation and maintenance scheme or the protocol PIMSM with “display Join/Prune” multicast routing formation and maintenance scheme is employed, there always exists a multicast application delay time for the process from initiating a specific multicast demand to receiving the needed multicast service flow.
In DVMRP and PIMDM protocols, provided the delay time for Graft message from leaf router to source-direct-connect router is Graft_Delay, and the transmission time for multicast data message from source to leaf router is M_Traffic_Delay, therefore we have: O<Delay≦Graft_Delay+M_Traffic_Delay, wherein Graft_Delay and M_Traffic_Delay are proportional to the transmission distance (the number of hops).
In PIMSM, assuming that the transmission delay time for Join message from leaf node to RP (Rendezvous Point) is Join_Delay; the transmission time for multicast data message from RP to leaf node is M_Traffic_Delay; the inhibition cycle of the register is T_Register (Time_Register), therefore the multicast application delay time of PIMSM satisfies the following relationship: O<Delay≦Join_Delay+T_Register+M_Traffic_Delay, wherein Join_Delay and M_Traffic_Delay are proportional to the distance (the number of hops) from the leaf node to RP. T_Register is related to the configuration of the Register timer, in which the default value is 60s. Because of said existing problems, if we use current multicast protocols, under the application environment which requires real-time processing, such as IPTV, the demands can not be met.